Resonator circuit

ABSTRACT

The invention relates to a resonator circuit, the resonator circuit comprising a transformer comprising a primary winding and a secondary winding, wherein the primary winding is inductively coupled with the secondary winding, a primary capacitor being connected to the primary winding, the primary capacitor and the primary winding forming a primary circuit, and a secondary capacitor being connected to the secondary winding, the secondary capacitor and the secondary winding forming a secondary circuit, wherein the resonator circuit has a common mode resonance frequency at an excitation of the primary circuit in a common mode, wherein the resonator circuit has a differential mode resonance frequency at an excitation of the primary circuit in a differential mode, and wherein the common mode resonance frequency is different from the differential mode resonance frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/235,474, filed on Apr. 20, 2021, which is a continuation of U.S.patent application Ser. No. 16/833,182, filed on Mar. 27, 2020, now U.S.Pat. No. 11,025,197, which is a continuation of U.S. patent applicationSer. No. 15/659,204, filed on Jul. 25, 2017, now U.S. Pat. No.10,637,395, which is a continuation of International Application No.PCT/EP2015/051573, filed on Jan. 27, 2015. All of the aforementionedpatent applications are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

The invention relates to the field of radio frequency (RF) resonatorcircuits.

BACKGROUND

Resonator circuits, also denoted as tank circuits, are widely used asfrequency selective elements in a variety of radio frequency components,such as filters, amplifiers, and oscillators. Typically, resonatorcircuits comprise inductors and capacitors, wherein the inductors andcapacitors are connected to be in resonance at a specific resonancefrequency. The quality of resonator circuits is thereby characterized bya quality factor. The characteristics of resonator circuits are of majorinterest in the design of radio frequency oscillators, in particularwhen implemented as radio frequency integrated circuits (RFICs) onsemiconductor substrates. In particular, the response of the resonatorcircuits with regard to leakage currents or currents at harmonicfrequencies can have a major impact on the frequency stability and phasenoise performance of the radio frequency oscillators.

Common resonator circuits exhibit a resistive characteristic whenexcited at the resonance frequency, and a capacitive characteristic whenexcited at frequencies above the resonance frequency. Consequently,higher order current components in conjunction with the Groszkowskieffect may lead to reduced frequency stability and increased flickernoise up-conversion, i.e. reduced close-in phase noise performance, ofradio frequency oscillators.

For improving the phase noise performance of radio frequencyoscillators, noise filtering techniques are applied. These techniquesrely on interposing a further resonator circuit having a resonancefrequency at 2ω₀ in a common source of the transistors, e.g. coretransistors. These techniques, however, use an additional tunableinductor and increase the die area on the semiconductor substrate.

For reducing an amount of higher order drain current harmonics,resistors are added in series with the sources of the transistors forlinearizing the operation of the transistors. However, the radiofrequency oscillator start-up margin is usually reduced.

By adding resistors in series with the drain of the transistors, theresistance in conjunction with a parasitic drain capacitance canintroduce a delay in a loop gain for shifting both an impulsesensitivity function (ISF) and a current waveform of the radio frequencyoscillators. Flicker noise up-conversion is reduced by specificallytailoring the component values. However, the phase noise performance inthe 20 dB/decade region is degraded particularly at low supply voltagesand high current consumptions.

In J. Groszkowski, “The interdependence of frequency variation andharmonic content, and the problem of constant-frequency oscillators,”Proc. IRE, vol. 21, no. 7, pp. 958-981, July 1934, the Groszkowskieffect is studied.

In M. Babaie and R. B. Staszewski, “A class-F CMOS oscillator,” IEEEJSSC, vol. 48, no. 12, pp. 3120-3133, December 2013, a resonator circuitand a radio frequency oscillator are described.

SUMMARY

It is an object of the invention to provide an efficient resonatorcircuit.

The invention is based on the finding that a transformer-based resonatorcircuit can be employed exhibiting different characteristics whenexcited in a differential mode and in a common mode. In particular, theinductive coupling factor of the transformer may be different indifferential mode and common mode excitations, wherein a differentialmode resonance frequency can be different from a common mode resonancefrequency. In particular, the common mode resonance frequency can bedesigned to be twice the differential mode resonance frequency.

The resonator circuit enables an efficient operation of a radiofrequency oscillator. In particular, a second harmonic can be exposed toa resistive path provided by the resonator circuit. Consequently, theGroszkowski effect can be mitigated and frequency stability and phasenoise performance of the radio frequency oscillator can be improved.

The resonator circuit and the radio frequency oscillator are suited forimplementation as radio frequency integrated circuits (RFICs) onsemiconductor substrates.

According to a first aspect, the invention relates to a resonatorcircuit, the resonator circuit comprising a transformer comprising aprimary winding and a secondary winding, wherein the primary winding isinductively coupled with the secondary winding, a primary capacitorbeing connected to the primary winding, the primary capacitor and theprimary winding forming a primary circuit, and a secondary capacitorbeing connected to the secondary winding, the secondary capacitor andthe secondary winding forming a secondary circuit, wherein the resonatorcircuit has a common mode resonance frequency at an excitation of theprimary circuit in a common mode, wherein the resonator circuit has adifferential mode resonance frequency at an excitation of the primarycircuit in a differential mode, and wherein the common mode resonancefrequency is different from the differential mode resonance frequency.Thus, an efficient resonator circuit is provided.

The resonator circuit can be a tank circuit. The resonator circuit canbe used as a frequency selective element within a radio frequencyoscillator. The resonator circuit can be resonant when excited in thedifferential mode and in the common mode.

The primary winding and the secondary winding can be arranged to providea strong inductive coupling when the primary circuit is excited in thedifferential mode and a weak inductive coupling when the primary circuitis excited in the common mode.

The primary capacitor can comprise a pair of single-ended capacitorsbeing connected in series to form the primary capacitor. The primarycapacitor can be regarded as a primary capacitive structure. Thesecondary capacitor can comprise a pair of differential capacitors beingconnected in series to form the secondary capacitor. The secondarycapacitor can be regarded as a secondary capacitive structure.

The resonance frequency in differential mode, i.e. the differential moderesonance frequency, can depend on the inductance of the primarywinding, the capacitance of the primary capacitor, the inductance of thesecondary winding, and the capacitance of the secondary capacitor. Theresonance frequency in common mode, i.e. the common mode resonancefrequency, can depend on the inductance of the primary winding and thecapacitance of the primary capacitor. The resonance frequency in commonmode, i.e. the common mode resonance frequency, may be independent fromthe inductance of the secondary winding, and the capacitance of thesecondary capacitor. Odd order harmonic components of the currentinjected into the resonator circuit can be differential mode signals andeven order harmonic components can be common mode signals.

In a first implementation form of the resonator circuit according to thefirst aspect as such, the common mode resonance frequency is twice thedifferential mode resonance frequency. Thus, a resistive path for asecond harmonic at an excitation of the primary circuit in the commonmode is realized.

In a second implementation form of the resonator circuit according tothe first aspect as such or any preceding implementation form of thefirst aspect, the resonator circuit has a further differential moderesonance frequency at an excitation of the primary circuit in thedifferential mode, wherein the further differential mode resonancefrequency is different from the differential mode resonance frequencyand the common mode resonance frequency. Thus, a further differentialmode resonance at the further differential mode resonance frequency isrealized.

In a third implementation form of the resonator circuit according to thesecond implementation form of the first aspect, the further differentialmode resonance frequency is three times the differential mode resonancefrequency. Thus, a resistive path for a third harmonic at an excitationof the primary circuit in the differential mode is realized. The furtherdifferential mode resonance frequency can specifically be designed to bethree times the differential mode resonance frequency.

In a fourth implementation form of the resonator circuit according tothe first aspect as such or any preceding implementation form of thefirst aspect, the primary winding of the transformer comprises one turn,and the secondary winding of the transformer comprises two turns. Thus,the resonator circuit is implemented efficiently.

The number of turns of the secondary winding can be twice the number ofturns of the primary winding. Thereby, a ratio of turns of the primarywinding and the secondary winding of 1:2 can be realized.

In a fifth implementation form of the resonator circuit according to thefirst aspect as such or any preceding implementation form of the firstaspect, the primary winding of the transformer and/or the secondarywinding of the transformer is planar. Thus, the resonator circuit isimplemented efficiently.

In a sixth implementation form of the resonator circuit according to thefirst aspect as such or any preceding implementation form of the firstaspect, the primary winding of the transformer and the secondary windingof the transformer are arranged on the same plane. Thus, the resonatorcircuit is implemented efficiently.

The primary winding of the transformer and/or the secondary winding ofthe transformer can comprise a bridging portion being arranged at adifferent plane.

In a seventh implementation form of the resonator circuit according tothe first aspect as such or any preceding implementation form of thefirst aspect, the primary winding of the transformer and/or thesecondary winding of the transformer is connected to a constant voltagesource or a ground potential. Thus, a tapping of the primary windingand/or the secondary winding is realized. Both the primary winding andthe secondary winding may be connected to a constant voltage sourcerespectively in order to enable an efficient start-up of the resonatorcircuit.

The tapping of the primary winding and/or the secondary winding can be asymmetrical center tapping of the primary winding and/or the secondarywinding.

In an eighth implementation form of the resonator circuit according tothe first aspect as such or any preceding implementation form of thefirst aspect, the primary capacitor of the primary circuit comprises apair of single-ended capacitors. Thus, a reference to ground potentialof the primary circuit is realized. The pair of single-ended capacitorscan have the same capacitance as the primary capacitor. A single-endedcapacitor can be realized as a plurality of switched capacitors, whereinthe plurality of switched capacitors can be arranged in parallel. Thecapacitances of the switched capacitors can be different.

In a ninth implementation form of the resonator circuit according to thefirst aspect as such or any preceding implementation form of the firstaspect, the secondary capacitor of the secondary circuit comprises apair of differential capacitors. Thus, a reference to ground potentialof the secondary circuit is avoided. A differential capacitor can berealized as a plurality of switched capacitors, wherein the plurality ofswitched capacitors can be arranged in parallel. The pair ofdifferential capacitors can be a pair of balanced capacitors.

In a tenth implementation form of the resonator circuit according to thefirst aspect as such or any preceding implementation form of the firstaspect, the primary capacitor and/or the secondary capacitor comprises avariable capacitor, in particular a digitally tunable capacitor. Thus, avariation of the differential mode resonance frequency and/or the commonmode resonance frequency can be realized efficiently.

In an eleventh implementation form of the resonator circuit according tothe first aspect as such or any preceding implementation form of thefirst aspect, the primary capacitor is connected in parallel to theprimary winding, and/or the secondary capacitor is connected in parallelto the secondary winding. Thus, the resonator circuit is implementedefficiently.

According to a second aspect, the invention relates to a radio frequencyoscillator, the radio frequency oscillator comprising a resonatorcircuit according to the first aspect as such or any implementation formof the first aspect, and an excitation circuit being configured toexcite the primary circuit of the resonator circuit in the differentialmode. Thus, an efficient radio frequency oscillator is provided.

The radio frequency oscillator can exhibit improved frequency stabilityand phase noise performance, e.g. close-in phase noise performance. Inparticular, flicker noise up-conversion due to the Groszkowski effectcan be mitigated efficiently. The approach may be effective to mitigatea 1/f phase noise up-conversion and may therefore improve a 1/f³ phasenoise characteristic. A 1/f² phase noise characteristic may beunchanged.

Further features of the radio frequency oscillator directly result fromthe functionality of the resonator circuit according to the first aspectas such or any implementation form of the first aspect.

In a first implementation form of the radio frequency oscillatoraccording to the second aspect as such, the excitation circuit comprisesat least one transistor, in particular at least one field-effecttransistor, for exciting the primary circuit of the resonator circuit.Thus, an active device is employed for exciting the primary circuit ofthe resonator circuit. In order to realize a cross-coupled oscillatorstructure at least two transistors may be employed. The transistors canbe metal-oxide-semiconductor field-effect transistors (MOSFETs).

In a second implementation form of the radio frequency oscillatoraccording to the second aspect as such or any preceding implementationform of the second aspect, the radio frequency oscillator is a class Foscillator. Thus, an efficient oscillator structure is applied.

Within the class F oscillator, a first harmonic and a third harmonic canbe excited in order to obtain a pseudo square-wave oscillation waveform.Within the class F oscillator, the third harmonic may not be filtereddue to the further differential mode resonance frequency. Theoscillation signal can have a pseudo square-wave oscillation waveform.

According to a third aspect, the invention relates to a method forexciting a resonator circuit, the resonator circuit comprising atransformer comprising a primary winding and a secondary winding,wherein the primary winding is inductively coupled with the secondarywinding, a primary capacitor being connected to the primary winding, theprimary capacitor and the primary winding forming a primary circuit, anda secondary capacitor being connected to the secondary winding, thesecondary capacitor and the secondary winding forming a secondarycircuit, wherein the resonator circuit has a common mode resonancefrequency at an excitation of the primary circuit in a common mode,wherein the resonator circuit has a differential mode resonancefrequency at an excitation of the primary circuit in a differentialmode, and wherein the common mode resonance frequency is different fromthe differential mode resonance frequency, the method comprisingexciting the primary circuit of the resonator circuit in thedifferential mode. Thus, an efficient excitation of the resonatorcircuit is realized.

The method can be performed by the resonator circuit and/or the radiofrequency oscillator. Further features of the method directly resultfrom the functionality of the resonator circuit and/or the radiofrequency oscillator.

The invention can be implemented using hardware and/or software.

SHORT DESCRIPTION OF DRAWINGS

Embodiments of the invention will be described with respect to thefollowing figures, in which:

FIG. 1 shows a diagram of a resonator circuit according to anembodiment;

FIG. 2 shows a diagram of a radio frequency oscillator according to anembodiment;

FIG. 3 shows an input impedance response and an equivalent circuit of aresonator circuit;

FIG. 4 shows an input impedance response and an equivalent circuit of aresonator circuit according to an embodiment;

FIG. 5 shows a transformer comprising a primary winding and a secondarywinding according to an embodiment;

FIG. 6 shows a resonator circuit and an input impedance responseaccording to an embodiment;

FIG. 7 shows a diagram of a radio frequency oscillator according to anembodiment;

FIG. 8 shows a diagram of a single-ended capacitor according to anembodiment;

FIG. 9 shows a diagram of a differential capacitor according to anembodiment;

FIG. 10 shows a diagram of a tail resistor of a radio frequencyoscillator according to an embodiment; and

FIG. 11 shows a phase noise power spectral density of a radio frequencyoscillator according to an embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a diagram of a resonator circuit 100 according to anembodiment. The resonator circuit 100 comprises a transformer 101comprising a primary winding 103 and a secondary winding 105, whereinthe primary winding 103 is inductively coupled with the secondarywinding 105, a primary capacitor 107 being connected to the primarywinding 103, the primary capacitor 107 and the primary winding 103forming a primary circuit, and a secondary capacitor 109 being connectedto the secondary winding 105, the secondary capacitor 109 and thesecondary winding 105 forming a secondary circuit, wherein the resonatorcircuit 100 has a common mode resonance frequency at an excitation ofthe primary circuit in a common mode, wherein the resonator circuit 100has a differential mode resonance frequency at an excitation of theprimary circuit in a differential mode, and wherein the common moderesonance frequency is different from the differential mode resonancefrequency. In an embodiment, the common mode resonance frequency istwice the differential mode resonance frequency.

The resonator circuit 100 can be a tank circuit. The resonator circuit100 can be used as a frequency selective element within a radiofrequency oscillator. The resonator circuit 100 can be resonant whenexcited in the differential mode and in the common mode.

The primary winding 103 and the secondary winding 105 can be arranged toprovide a strong inductive coupling when the primary circuit is excitedin the differential mode and a weak inductive coupling when the primarycircuit is excited in the common mode.

The resonance frequency in differential mode, i.e. the differential moderesonance frequency, can depend on the inductance of the primary winding103, the capacitance of the primary capacitor 107, the inductance of thesecondary winding 105, and the capacitance of the secondary capacitor109. The resonance frequency in common mode, i.e. the common moderesonance frequency, can depend on the inductance of the primary winding103 and the capacitance of the primary capacitor 107. The resonancefrequency in common mode, i.e. the common mode resonance frequency, maybe independent from the inductance of the secondary winding 105, and thecapacitance of the secondary capacitor 109.

The diagram illustrates the overall structure of the resonator circuit100, wherein the primary capacitor 107 can comprise a pair ofsingle-ended capacitors, and wherein the secondary capacitor 109 cancomprise a pair of differential capacitors.

FIG. 2 shows a diagram of a radio frequency oscillator 200 according toan embodiment. The radio frequency oscillator 200 comprises a resonatorcircuit 100, and an excitation circuit 201. The resonator circuit 100comprises a transformer 101 comprising a primary winding 103 and asecondary winding 105, wherein the primary winding 103 is inductivelycoupled with the secondary winding 105, a primary capacitor 107 beingconnected to the primary winding 103, the primary capacitor 107 and theprimary winding 103 forming a primary circuit, and a secondary capacitor109 being connected to the secondary winding 105, the secondarycapacitor 109 and the secondary winding 105 forming a secondary circuit,wherein the resonator circuit 100 has a common mode resonance frequencyat an excitation of the primary circuit in a common mode, wherein theresonator circuit 100 has a differential mode resonance frequency at anexcitation of the primary circuit in a differential mode, and whereinthe common mode resonance frequency is different from the differentialmode resonance frequency. The excitation circuit 201 is configured toexcite the primary circuit of the resonator circuit 100 in thedifferential mode. The excitation circuit 201 can exemplarily comprisean exciting element, e.g. a feedback amplifier, providing atrans-conductance Gm.

In an embodiment, the excitation circuit 201 comprises at least onetransistor, in particular at least one field-effect transistor, forexciting the primary circuit of the resonator circuit 100. In order torealize a cross-coupled oscillator structure at least two transistorsmay be employed.

In the following, further implementation forms and embodiments of theresonator circuit 100 and the radio frequency oscillator 200 aredescribed.

An up-conversion of flicker noise, e.g. 1/f noise, can degrade aclose-in spectrum of a radio frequency oscillator, e.g. a complementarymetal-oxide semiconductor (CMOS) radio frequency (RF) oscillator. Theresulting 1/f³ phase noise (PN) can further be an issue withinphase-locked loops (PLLs) having a loop bandwidth of e.g. less than 1MHz, which practically relates to the majority of cellular phones. Amajor flicker noise up-conversion mechanism in nanoscale CMOS is theGroszkowski effect.

The presence of harmonics in a current of an active device, such as atransistor of an excitation circuit, can cause a frequency drift of aresonance frequency of a resonator circuit, due to perturbing reactiveenergy in the resonator circuit. Any variation in the ratio of a higherharmonic current to a fundamental current (e.g. due to the flickernoise) can modulate the frequency drift and can show itself as a 1/f³phase noise. Embodiments of the invention reduce the flicker noiseup-conversion due to the Groszkowski effect in radio frequencyoscillators significantly. The resonator circuit 100 can be applied forflicker noise up-conversion reduction within the radio frequencyoscillator 200, wherein the radio frequency oscillator 200 can be aclass F oscillator.

FIG. 3 shows an input impedance response 301 and an equivalent circuit303 of a resonator circuit. The diagram illustrates current harmonicpaths and frequency drifts for the resonator circuit without resistivetraps at higher harmonics.

The presence of harmonics of a current of an active device, such as atransistor of an excitation circuit, can cause a frequency drift of aresonance frequency ω₀ of a resonator circuit as depicted in FIG. 3 . Afundamental drain current I_(H1) can flow into the resistors having theresistance R_(p), which can be the equivalent parallel resistance of theresonator circuit, while its second and third harmonics, I_(H2) andI_(H3), may mainly take the capacitance path due to its lower impedance.As a consequence, reactive energy stored in the inductors andcapacitors, e.g. having inductances L_(p) and capacitances C_(c) andC_(d), can be perturbed, shifting the resonance frequency ω₀ and/or theoscillation frequency by Δω lower in order to satisfy the resonancecondition. This shift may be static but any variation in the ratio ofthe currents I_(H2) (or I_(H3)) to I_(H1) (e.g. due to flicker noise)can modulate Δω and can show itself as a 1/f³ phase noise.

FIG. 4 shows an input impedance response 401 and an equivalent circuit403 of a resonator circuit 100 according to an embodiment. The diagramillustrates current harmonic paths and frequency drifts for theresonator circuit 100 with resistive traps at higher harmonics.

Suppose that the input impedance Z_(in) of the resonator circuit 100 hasfurther peaks at strong harmonics of the fundamental resonance frequencyω₀. These harmonics would then mainly flow into their relativeequivalent resistance of Z_(in), instead of its capacitive part, asdepicted in FIG. 4 . Consequently, Groszewski's effect on the flickernoise up-conversion can be reduced significantly. On the other hand,flicker noise of transistors, e.g. core transistors, of an excitationcircuit can modulate the second harmonic of the virtual ground of theradio frequency oscillator. This modulation can generate a secondharmonic current in parasitic gate-source capacitors C_(gs) and can getinjected into the resonator circuit 100. Consequently, the currentI_(H2) can be the main contributor to the frequency drift.

In other words, a dominant source of 1/f noise up-conversion in radiofrequency oscillators, in particular without tail transistors, is thatcurrent harmonics of the resonator circuits flow into the capacitivepart of the resonator circuits as shown in FIG. 3 . An approach forreducing the 1/f noise up-conversion is illustrated in FIG. 4 . Forresonance frequencies at higher harmonics, the current can flow into theequivalent resistance of the resonator circuit and 1/f noiseup-conversion can be reduced. A resonator circuit 100 is generally shownin FIG. 1 .

Embodiments of the invention apply a transformer-based resonator circuittopology that effectively traps the current I_(H2) in its resistive partwithout the cost of extra die area on a semiconductor substrate. Theresonator circuit 100 can derive this characteristic from a differentbehavior of inductors and transformers in differential mode (DM) andcommon mode (CM) excitations. The transformer based resonator circuit100 can be incorporated into the radio frequency oscillator 200, e.g. aclass-F oscillator, in order to take advantage of its low phase noise inthe 20 dB/dec region and in order to improve the phase noise in the 30dB/dec region.

The resonator circuit 100 can be based on the transformer 101, e.g.being a 1:2 turn transformer. The differential mode resonance frequencyand the common mode resonance frequency can be different within thetransformer 101, e.g. due to different coupling factors in differentialmode and in common mode. An application of a switch is avoided. Theresistive trap is realized by the common mode resonance.

The common mode signal that excites the common mode resonance can be thesecond harmonic component of the current within the resonator circuit100. The I_(H2) component can have a n/2 phase shift with regard to thefundamental current which can make it a common mode signal asillustrated in FIG. 3 and FIG. 4 .

If the space of the primary winding and/or the secondary winding isdesigned accurately and a ratio C_(s)/C_(p) is chosen accurately, thecommon mode resonance frequency can be two times the differential moderesonance frequency. Then, the common mode second harmonic currentcomponent can flow into the equivalent resistance of the resonant peakand may not flow through the capacitive part. This approach mitigatesdisturbances of the reactive energy in the capacitive part and reducesthe 1/f noise up-conversion.

FIG. 5 shows a transformer 101 comprising a primary winding 103 and asecondary winding 105 according to an embodiment. The diagramillustrates the currents within the primary winding 103 and thesecondary winding 105 when the transformer 101 is excited indifferential mode (DM) and in common mode (CM). The transformer 101 canbe an F_(2,3) transformer.

The primary winding 103 of the transformer 101 and the secondary winding105 of the transformer 101 are planar and are arranged on the sameplane. The secondary winding 105 of the transformer 101 comprises abridging portion being arranged at a different plane.

The primary winding 103 of the transformer 101 and the secondary winding105 of the transformer 101 are connected to a supply voltage or analternating current (AC) ground potential. The connection is realized bya symmetrical center tapping of the primary winding 103 and thesecondary winding 105.

The transformer 101, having a 1:2 turn ratio, can be excited bydifferential mode and common mode input signals at its primary winding103. In differential mode excitation, the induced currents at thesecondary winding 105 can circulate in the same directions leading to astrong coupling factor k_(m). On the other hand, in common modeexcitation, the induced currents can cancel each other, resulting in aweak coupling factor k_(m).

The inductance of the primary winding 103 can be referred to as L_(p),the inductance of the secondary winding 105 can be referred to as L_(s),the capacitance of the primary capacitor 107 can be referred to asC_(p), and the capacitance of the secondary capacitor 109 can bereferred to as C_(s). According to this definition, the primary winding103, the secondary winding 105, the primary capacitor 107, and thesecondary capacitor 109 are considered as individual concentratedcomponents.

Alternatively, the inductance of the primary winding 103 can be referredto as 2 L_(p), the inductance of the secondary winding 105 can bereferred to as 2 L_(s), the capacitance of the primary capacitor 107 canbe referred to as 0.5 C_(p), and the capacitance of the secondarycapacitor 109 can be referred to as 0.5 C_(s). According to thisdefinition, the primary winding 103 and the secondary winding 105 areeach formed by a pair of inductors connected in series, wherein theinductance of each inductor is referred to as L_(p) or L_(s),respectively. Furthermore, the primary capacitor 107 and the secondarycapacitor 109 are each formed by a pair of capacitors connected inseries, wherein the capacitance of each capacitor is referred to asC_(p) or C_(s), respectively.

The differential mode resonance frequency can be determined according tothe following equation:

$\omega_{0,{DM}} = \frac{1}{\sqrt{{L_{pd}C_{p}} + {L_{s}C_{s}}}}$wherein ω_(0,DM) denotes the differential mode resonance frequency,L_(pd) denotes an inductance associated with the primary winding 103 indifferential mode, C_(p) denotes a capacitance associated with a primarycapacitor 107, L_(s) denotes an inductance associated with the secondarywinding 105, and C_(s) denotes a capacitance associated with a secondarycapacitor 109.

The common mode resonance frequency can be determined according to thefollowing equation:

$\omega_{CM} = \frac{1}{\sqrt{L_{pc}C_{p}}}$wherein ω_(CM) denotes the common mode resonance frequency, L_(pc)denotes an inductance associated with the primary winding 103 in commonmode, and C_(p) denotes a capacitance associated with a primarycapacitor 107.

L_(pd) can relate to half of the inductance of the primary winding 103in differential mode, e.g. the inductance between a center tap of theprimary winding 103 and one of the inputs, yielding a total differentialprimary capacitance of 2 L_(p). This is due to the consideration thatthe inductance L_(T) may not be seen in differential excitation but mayaffect the inductance in common mode excitation. The total inductance incommon mode excitation can be equal to 2 L_(pd)+2 L_(T), orL_(pc)=L_(pd)+L_(T) as used in the equations.

In an embodiment, the inductance associated with the primary winding 103in differential mode and the inductance associated with the primarywinding 103 in common mode are considered to be equal.

FIG. 6 shows a resonator circuit 100 and an input impedance response 609according to an embodiment. The resonator circuit 100 comprises atransformer 101 comprising a primary winding 103 and a secondary winding105, wherein the primary winding 103 is inductively coupled with thesecondary winding 105, a primary capacitor 107 being connected to theprimary winding 103, the primary capacitor 107 and the primary winding103 forming a primary circuit, and a secondary capacitor 109 beingconnected to the secondary winding 105, the secondary capacitor 109 andthe secondary winding 105 forming a secondary circuit, wherein theresonator circuit 100 has a common mode resonance frequency at anexcitation of the primary circuit in a common mode, wherein theresonator circuit 100 has a differential mode resonance frequency at anexcitation of the primary circuit in a differential mode, and whereinthe common mode resonance frequency is different from the differentialmode resonance frequency. FIG. 6 shows a possible realization of theresonator circuit 100.

The primary capacitor 107 of the primary circuit comprises a pair ofsingle-ended capacitors 601, 603. The secondary capacitor 109 of thesecondary circuit comprises a pair of differential capacitors 605, 607.The primary capacitor 107 and the secondary capacitor 109 are variablecapacitors, in particular digitally tunable capacitors. In particular,the pair of single-ended capacitors 601, 603 and the pair ofdifferential capacitors 605, 607 are variable capacitors, in particulardigitally tunable capacitors. The differential mode resonance frequencyand/or the common mode resonance frequency are tunable between a minimumfrequency f_(min) and a maximum frequency f_(max), respectively, asillustrated by the input impedance response 609. The input impedance ofthe resonator circuit 100 is denoted as Z_(in).

The resonator circuit 100 can employ the transformer 101, the pair ofsingle-ended capacitors 601, 603 within the primary circuit and the pairof differential capacitors 605, 607 within the secondary circuit. Theresonator circuit 100 can be an F_(2,3) resonator circuit. Thetransformer 101 can be an F_(2,3) transformer. The resonator circuit 100can have two differential mode resonance frequencies and one common moderesonance frequency.

For class-F₃ operation, ω_(1,DM)=3ω_(0,DM), and for resistive traps atthe second and third harmonics, ω_(CM)=2ω_(0,DM) and ω_(1,DM)=3ω_(0,DM).This can result in L_(s)C_(s)=3L_(p)C_(p) and k_(m)=0.72, wherein k_(m)denotes the coupling factor between the primary winding 103 and thesecondary winding 105.

When implementing the resonator circuit 100, the inductance associatedwith the primary winding 103 in common mode L_(pc) can be greater thanthe inductance associated with the primary winding 103 in differentialmode L_(pd), i.e. L_(pc)>L_(pd), due to a metal track inductance L_(T)connecting e.g. a center tap of the primary winding 103 to a constantsupply voltage. Thus, a lower coupling factor k_(m) may be used in orderto satisfy both F₂ and F₃ operation conditions of the resonator circuit100. A careful design of the single-ended capacitors 601, 603 within theprimary circuit and/or the differential capacitors 605, 607 within thesecondary circuit, which can be variable capacitors, can maintainω_(CM)/ω_(0,DM)≈2 and ω_(1,DM)/ω_(0,DM)≈3 over the full tuning range(TR).

In an embodiment, the inductance associated with the primary winding 103in common mode L_(pc) is determined according to the following equation:L _(pc) =L _(pd) +L _(T)wherein L_(pc) denotes the inductance associated with the primarywinding 103 in common mode, L_(pd) denotes the inductance associatedwith the primary winding 103 in differential mode, and L_(T) denotes themetal track inductance.

FIG. 7 shows a diagram of a radio frequency oscillator 200 according toan embodiment. The radio frequency oscillator 200 comprises a resonatorcircuit 100, and an excitation circuit 201. The resonator circuit 100forms an implementation of the resonator circuit 100 as described inconjunction with FIG. 6 . The excitation circuit 201 comprises atransistor 701, a transistor 703, a tail resistor 705, and a tailcapacitor 707. The radio frequency oscillator 200 is a class Foscillator.

Class F₃ oscillators can have a pseudo square-wave oscillation waveformby designing ω_(1,DM)=3_(ω0,DM), and avoiding filtering the currentI_(H3) in a resonator circuit. The specific impulse sensitivity function(ISF) of the pseudo square-wave oscillation waveform can lead to animproved phase noise performance. In this oscillator, the current I_(H2)can be as high as the current I_(H3). In a class F_(2,3) oscillator, aclass F₃ resonator circuit is replaced by a class F_(2,3) resonatorcircuit. The pseudo square-wave oscillation waveform of class Foscillators can be preserved, wherein a 1/f³ phase noise cornerfrequency can be reduced e.g. from 300 kHz to 700 kHz to less than 30kHz. Embodiments of the invention use an F_(2,3) resonator circuit andthe different characteristics of a 1:2 turn transformer in differentialmode and common mode excitations in order to provide a resistive trap atthe second harmonic 2ω₀, resulting in a reduction of flicker noiseup-conversion in radio frequency oscillators.

FIG. 8 shows a diagram of a single-ended capacitor 601, 603 according toan embodiment. The single-ended capacitor 601, 603 comprises a capacitor801, a capacitor 803, a transistor 805, a transistor 807, a resistor809, a resistor 811, an inverter 813, and an inverter 815. Thesingle-ended capacitor 601, 603 is arranged within the primary circuit.

By applying a digital switching signal the transistor 805 and thetransistor 807 can be switched between a conducting state and anon-conducting state. Consequently, the capacitance of the single-endedcapacitor 601, 603 can be digitally tuned. A plurality of single-endedcapacitors 601, 603 can be connected in parallel.

FIG. 9 shows a diagram of a differential capacitor 605, 607 according toan embodiment. The differential capacitor 605, 607 comprises a capacitor901, a capacitor 903, a transistor 905, a resistor 907, a resistor 909,an inverter 911, and an inverter 913. The differential capacitor 605,607 is arranged within the secondary circuit.

By applying a digital switching signal the transistor 905 can beswitched between a conducting state and a non-conducting state.Consequently, the capacitance of the differential capacitor 605, 607 canbe digitally tuned. A plurality of differential capacitors 605, 607 canbe connected in parallel.

FIG. 10 shows a diagram of a tail resistor 705 of a radio frequencyoscillator 200 according to an embodiment. The tail resistor 705comprises a transistor 1001, and a resistor 1003. The tail resistor 705can be used for current control within the radio frequency oscillator200.

By applying a digital switching signal b_(i), the transistor 1001 can beswitched between a conducting state and a non-conducting state.Consequently, the current within the radio frequency oscillator 200 canbe controlled. A plurality of tail resistors 705 can be connected inparallel and/or in series.

FIG. 11 shows a phase noise power spectral density 1101 of a radiofrequency oscillator 200 according to an embodiment. The diagram depictsthe phase noise power spectral density in dBc/Hz over a carrierfrequency offset in Hz. The radio frequency oscillator 200 is a classF_(2,3) oscillator.

The diagram relates to a minimum frequency of 5.4 GHz and a maximumfrequency of 7 GHz. A 1/f³ phase noise corner is further depicted in thediagram.

It will be appreciated that statements made herein characterizing theinvention refer to an embodiment of the invention and not necessarilyall embodiments.

The invention claimed is:
 1. A capacitor for tuning a differential moderesonance frequency of a radio frequency oscillator, comprising: aprimary capacitor; and a secondary capacitor; wherein the primarycapacitor comprises a pair of single-ended capacitors; wherein thesecondary capacitor comprises a pair of differential capacitorsconnected in series; wherein the primary capacitor and the secondarycapacitor are variable, and are configured to tune the differential moderesonance frequency and a common mode resonance frequency of the radiofrequency oscillator; wherein the common mode resonance frequency of theradio frequency oscillator is independent from the capacitance of thesecondary capacitor.
 2. The capacitor of claim 1, wherein the primarycapacitor is connected to two ends of a primary winding and thesecondary capacitor is connected to two ends of a secondary winding. 3.The capacitor of claim 2, wherein the pair of single-ended capacitorscomprises a first capacitor and a second capacitor; wherein a first endof the first capacitor is coupled to a first end of the primary windingand a second end of the first capacitor is coupled to a groundpotential; wherein a first end of the second capacitor is coupled to asecond end of the primary winding and a second end of the secondcapacitor is coupled to the ground potential.
 4. The capacitor of claim3, wherein the pair of single-ended capacitors further comprises a firsttransistor and a second transistor; wherein the first transistor isconfigured to connect and disconnect the first capacitor between thefirst end of the primary winding and the ground potential; wherein thesecond transistor is configured to connect and disconnect the secondcapacitor between the second end of the primary winding and the groundpotential.
 5. The capacitor of claim 4, wherein a gate of the firsttransistor and a gate of the second transistor are coupled to a samecontrol signal from an inverter.
 6. The capacitor of claim 1, whereinthe pair of differential capacitors comprises a first differentialcapacitor and a second differential capacitor.
 7. The capacitor of claim6, wherein the first differential capacitor and the second differentialcapacitor are a pair of balanced capacitors.
 8. The capacitor of claim7, further comprising: a switch coupled between the first differentialcapacitor and the second differential capacitor, wherein the switch isconfigured to connect and disconnect the first differential capacitorand the second differential capacitor to and from one another.
 9. Thecapacitor of claim 1, wherein the pair of differential capacitorscomprises a plurality of switched capacitors configurable to be coupledin parallel.
 10. The capacitor of claim 1, wherein the radio frequencyoscillator comprises a resistor for current control within the radiofrequency oscillator.
 11. The capacitor of claim 10, wherein theresistor is tunable.
 12. The capacitor of claim 1, wherein the primarycapacitor is connected to two ends of a primary winding, and wherein theinductance associated with the primary winding in the common mode isgreater than the inductance associated with the primary winding in thedifferential mode.